The world's energy demand are ever increasing and the fossil fuel sources are being depleted, leading to increasing competition for the available energy sources, and thereby hampering economic growth by high energy prices. To overcome this situation renewable energy resources such as wind, solar and biomass must be brought into exploitation.
The rapid increase in global energy demand makes the search for new energy sources a major concern, and as a consequence, there is a big interest in the production and use of renewable energy.
Biomass and waste in various forms is a huge source of renewable energy, and utilization of biomass, in contrast to utilization of fossil resources such as oil and gas, does not cause a net increase in atmospheric CO2. The increasing level of CO2 in the atmosphere is by most scholars considered to cause global warming (e.g. Karl and Trenberth, 2003). There is globally a large and increasing demand for liquid fuels produced from biomass, not only for environmental reasons, but also for pure economical ones due to increasing oil prices and security of oil supply (International Energy Outlook, 2008).
Several processes for thermal transformation of biomass to liquids with fuel properties exists, however most of them are carried out on dry biomass such as pyrolysis. Pyrolysis is the process of thermochemical transformation of biomass under non-oxidative conditions (e.g. Yaman, 2004). Typical pyrolysis conditions are 500-520° C. for most forms of woody biomass (Demirbas, 2007). Other thermal processes for liquid fuel production include Catalytic depolymerization (CDP) and biomass gasification combined with Fischer-Tropsch synthesis (BG-FT) (Laohalidanond et al., 2006)
Many of the above mentioned processes are not optimal for wet feeds. However, many biomass based materials, such as wet grains from ethanol or beer production, bagasse from sugar production, or sludges such as manure fibres from live stock production contain a large fraction of water. To remove the water by evaporation consumes a considerable amount of energy. In hydrothermal gasification and liquefaction processes aqueous biomass is directly converted to oil, water soluble organic substances, gas and minerals at sub- or supercritical critical conditions (Srokol et al., 2004; Karagöz et al., 2006; Elliott et al., 1991; Sealock et al., 1993).
The presence of alkali's such as potassium and sodium are well known to be beneficial for degradation and conversion of organic macromolecules in the feed. Such alkalis are often present in the organic feed and sometimes additional alkalis are added to the feed to enhance the conversion (Iversen et al, 2006, Zhong et al., 2002; Feng et al., 2004; Karagöz et al., 2005).
Iversen et al (WO2006/1170002A3) further control the pH of the of feed during the conversion process, and apply a heterogeneous catalyst to further control the conversion process to oil.
However, common for such hydrothermal conversion processes is that at least part of the organics in the feed ends up as water soluble organics in the water effluent. Typical such water soluble organics may comprises alcohols, phenols, amines, amides, acids, ketones, ethers, esters and aldehydes of c1-c10 hydrocarbons. Depending on the specific process configuration, feed and operating conditions up a significant amount of the carbon and energy content in the feed may end up as water soluble organics. Thereby a polluted water effluent is created, which needs to be purified in order to provide an environmentally sustainable process. Further it is advantageous and very desirably to recover these water soluble organics as the process becomes more efficient both energy wise and economically.
As mentioned above the presence of alkali's such as potassium and/or sodium are beneficial for the conversion process. Alkali's are often present in feeds relevant for the present invention, but it is often desired to add additional alkali's for further enhancing the conversion process. However, alkali salts are relatively expensive, and constitute a major operating cost of conversion process. Hence, it is desirable to at least partly recover alkali salts from the process.
Hence an improved conversion method and apparatus would be advantageous.